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Abstract:

A surface protective film includes: a polyolefin base material layer; and
an adhesion layer, in which the adhesion layer includes: a
vinyl-polyisoprene-block-containing styrene elastomer; a polyolefin; and
a tackifier, and a hardness (Martens hardness) of the adhesion layer is
1.0 N/mm2 or more and 2.5 N/mm2 or less.

Claims:

1. A surface protective film comprising: a polyolefin base material
layer; and an adhesion layer, wherein the adhesion layer comprises: a
vinyl-polyisoprene-block-containing styrene elastomer; a polyolefin; and
a tackifier, and a hardness (Martens hardness) of the adhesion layer is
1.0 N/mm2 or more and 2.5 N/mm2 or less.

2. The surface protective film according to claim 1, wherein a mass ratio
of the polyolefin relative to the vinyl-polyisoprene-block-containing
styrene elastomer is 0.1 or more and 2 or less, and a mass ratio of the
tackifier relative to the vinyl-polyisoprene-block-containing styrene
elastomer is 0.03 or more and 1 or less.

3. The surface protective film according to claim 1, wherein the styrene
elastomer comprises styrene at 5 mass % or more and 30 mass % or less and
triblock copolymer at 80 mass % or more, and a glass transition
temperature of the styrene elastomer is -40 degrees C. or more and 20
degrees C. or less.

4. The surface protective film according to claim 1, wherein, the
thickness of the adhesion layer is 5 μm or more and 25 μm or less.

5. The surface protective film according to claim 1, wherein, the
adhesion layer is laminated by co-extrusion.

Description:

TECHNICAL FIELD

[0001] The present invention relates to a surface protective film. In
particular, the present invention relates to a surface protective film
favorable for protecting a surface of an adherend having an irregular
surface such as a prism sheet.

BACKGROUND ART

[0002] A surface protective film that is temporarily attached to a surface
of a metal plate, a glass plate or a synthetic resin plate for preventing
the surface from being damaged or being attached with dust has been known
(e.g., Patent Literature 1 to Patent Literature 3).

[0003] For instance, Patent Literature 1 discloses a surface protective
film formed by co-extruding a polyolefin resin base material and an
adhesive layer, the adhesive layer including a composition that contains
a block copolymer represented by a general formula A-B-A (A is a styrene
copolymer block and B is a copolymer block of ethylene and butylene) and
a tackifier resin.

[0004] The surface protective film disclosed in Patent Literature 1 can be
laminated by co-extrusion and allows an adhesive to become solvent-free,
so that the surface protective film is excellent in terms of
environmental protection and safety and hygiene during manufacturing.
Further, the surface protective film is economically favorable because
its manufacturing process can be simplified.

[0008] However, when a typical surface protective film as disclosed in
Patent Literature 1 is rolled up and stored or transferred as a roll, an
adhesive layer wound around a rear surface of a base material may firmly
adhere to the base material.

[0009] Consequently, the surface protective film is not easily wound off
from the roll, reducing efficiency of operation for attaching to a
surface of an adherend. When the surface protective film is forcefully
wound off, the base material may be partially stretched and deformed or
the adhesive layer may be partially peeled off, so that the surface
protective film may not function.

[0010] To solve these problems, a mold releasing agent is applied to or a
mold releasing treatment such as corona treatment is applied to the rear
surface of the base material.

[0011] However, in use of a mold releasing agent, a particular kind of
mold releasing agent that firmly adheres to the base material but does
not adhere to the adhesive layer is required. In addition, when the mold
releasing agent seeps into the adhesive layer, the adhesive force of the
surface protective film to the adherend may be reduced.

[0012] Further, due to a complicated process for applying a mold releasing
agent or effecting a mold releasing treatment such as corona treatment,
manufacturing cost of the surface protective film may be increased.

[0013] In order to solve the above problems, a method for controlling an
adhesion strength has also been typically used. Such a method for
controlling an adhesion strength is exemplified by a method disclosed in,
for instance, Patent Literature 2, in which the amount of diblock in a
styrene elastomer is adjusted to control an adhesion strength. In Patent
Literature 3, an adhesion strength is controlled by adjusting the mixed
amount of a tackifier.

[0014] However, in the method of Patent Literature 2, an increase in the
amount of diblock, which can increase the adhesion strength, makes the
film too sticky to unroll from the roll.

[0015] In the method of Patent Literature 3, an adjustable range of the
mixed amount is too narrow to achieve both adhesion strength and
unwinding property from the roll.

[0016] An object of the invention is to provide a surface protective film:
being capable of solving the above problems; being moldable by
co-extrusion; having an adhesion strength controllable by appropriately
adjusting the hardness of an adhesion layer within a predetermined range;
exhibiting an excellent unwinding property from a roll without applying a
separating agent to a surface of a base material layer; and being capable
of maintaining long-lasting stable adhesive force and unwinding property.
Further, an object of the invention is to provide a surface protective
film having an adhesion strength adjustable based on the hardness of at
least either one of a styrene elastomer or a polyolefin contained in the
adhesion layer.

Means for Solving the Problems

[0017] According to an aspect of the invention, a surface protective film
including: a polyolefin base material layer; and an adhesion layer, in
which the adhesion layer includes: a vinyl-polyisoprene-block-containing
styrene elastomer; a polyolefin; and a tackifier, and a hardness (Martens
hardness) of the adhesion layer is 1.0 N/mm2 or more and 2.5
N/mm2 or less.

[0018] In the surface protective film, it is preferable that a mass ratio
of the polyolefin relative to the vinyl-polyisoprene-block-containing
styrene elastomer is 0.1 or more and 2 or less, and a mass ratio of the
tackifier relative to the vinyl-polyisoprene-block-containing styrene
elastomer is 0.03 or more and 1 or less.

[0019] In the surface protective film, it is preferable that the styrene
elastomer includes styrene at 5 mass % or more and 30 mass % or less and
triblock copolymer at 80 mass % or more, and a glass transition
temperature of the styrene elastomer is -40 degrees C. or more and 20
degrees C. or less.

[0020] In the surface protective film, it is preferable that the thickness
of the adhesion layer is 5 μm or more and 25 μm or less.

[0021] In the surface protective film, it is preferable that the adhesion
layer is laminated by co-extrusion.

[0022] With the above arrangement, it is possible to provide a surface
protective film: being moldable by co-extrusion; having an adhesion
strength controllable by appropriately adjusting the hardness of an
adhesion layer within a predetermined range; exhibiting an excellent
unwinding property from a roll without applying a separating agent to a
surface of a base material layer; and being capable of maintaining
long-lasting stable adhesive force and unwinding property. Further, it is
possible to provide a surface protective film having an adhesion strength
adjustable based on the hardness of at least either one of a styrene
elastomer or a polyolefin contained in the adhesion layer.

Exemplary Embodiment of the Invention

[0023] An exemplary embodiment of the invention will be described below.

[0024] A surface protective film of the exemplary embodiment includes a
base material layer and an adhesion layer.

[0025] The base material layer is not particularly limited as long as it
is a sheet or film that is commonly used as a support of a surface
protective film, examples of which include a film formed of a polyolefin
material or the like.

[0026] Examples of polyolefin usable for the base material layer include
low-density polyethylene, medium-density polyethylene, high-density
polyethylene, linear low-density polyethylene, ethylene-α-olefin
copolymer, ethylene-vinyl-acetate copolymer, ethylene-ethylacrylate
copolymer, ethylene-methyl methacrylate copolymer, ethylene-n-butyl
acrylate copolymer, and polypropylene (homopolymer, random copolymer,
block copolymer). The resins as described above may be used alone or in
combination. Especially, block copolymer polypropylene (BPP) is favorable
as a material for the base material layer. The surface of the base
material layer is roughened by using BPP and thus unwinding property is
further enhanced while tearing strength and impact strength are also
enhanced. Further, heat resistance and rigidity can be added.

[0027] Additives such as a pigment, an antioxidant, a stabilizer and an
ultraviolet absorber may be added to the base material layer as needed.

[0028] The base material layer may be formed of a plurality of layers. The
total thickness of the base material layer formed of a single layer or a
plurality of layers is favorably 30 μm or more and 80 μm or less.

[0029] The adhesion layer contains a vinyl-polyisoprene-block-containing
styrene elastomer, a polyolefin; and a tackifier.

[0030] The vinyl-polyisoprene-block-containing styrene elastomer contained
in the adhesion layer is a block copolymer represented by a general
formula (1) or general formula (2).

Formula 1

A-B-A (1)

A-B (2)

[0031] In the general formulae (1) and (2), A is a styrene block and B is
a non-hydrogenated vinyl-polyisoprene block or hydrogenated
vinyl-polyisoprene block represented by a general formula (3) as follows.
The styrene elastomer contains an uncoupled styrene block and an
uncoupled hydrogenated or non-hydrogenated vinyl-polyisoprene block.

##STR00001##

[0032] In the vinyl-polyisoprene-block-containing styrene elastomer, the
styrene content is 5 mass % or more and 30 mass % or less, preferably 10
mass % or more and 25 mass % or less. When the styrene content is less
than 5 mass %, the hardness is low and the adhesion strength is too high.
Thus, adhesive deposits may be caused on the adherend after peeling.
Especially, adhesive deposits are easily caused on an adherend having an
irregular surface such as a prism sheet. When the styrene content exceeds
30 mass %, the hardness is too high and thus the adhesion strength to the
adherend is weak, so that the surface protective film may be peeled off.
Especially, the surface protective film is easily peeled off from an
adherend having an irregular surface such as a prism sheet.

[0033] Here, the styrene content means a styrene block content relative to
the total amount of the vinyl-polyisoprene-block-containing styrene
elastomer.

[0034] The styrene content is calculated by, for instance, the following
method.

[0035] A block copolymer composition contained in the adhesion layer of
the protective film is dissolved in a small amount of hexane and, then,
an excessive amount of acetone is added thereto, so that the composition
is separated into acetone-insoluble components and acetone-soluble
components. The insoluble components are measured by an NMR (nuclear
magnetic resonance spectrum method) so as to calculate the styrene
content from an integral intensity ratio of spectrum.

[0036] A triblock copolymer is a block copolymer represented by the above
general formula (1).

[0037] The triblock copolymer content is 80 mass % or more, preferably 90
mass % or more. When the triblock copolymer content is less than 80 mass
%, the adhesion strength to the adherend is too high and thus adhesive
deposits may be left after peeling. Also, since the adhesion strength to
the rear surface of the base material layer is increased, unwinding
property from the roll may be reduced.

[0038] The triblock copolymer content is calculated by, for instance, the
following method.

[0039] The block copolymer composition contained in the adhesion layer of
the protective film is dissolved in tetrahydrofuran (TFH). Two columns
for liquid chromatography of GS5000H and two columns for liquid
chromatography of G4000H (four columns in total) manufactured by Tosoh
Corporation are connected in series. High-performance liquid
chromatography is performed at a temperature of 40 degrees C. and a flow
rate of 1 ml/min by using THF for a mobile phase. From the obtained
chart, a coupling component, i.e., a peak area corresponding to a
triblock copolymer, is obtained. A percentage of the peak area relative
to an entire peak area is the triblock copolymer content.

[0040] The glass transition temperature of the
vinyl-polyisoprene-block-containing styrene elastomer is -40 degrees C.
or more and 20 degrees C. or less. When the glass transition temperature
of the vinyl-polyisoprene-block-containing styrene elastomer is less than
-40 degrees C., the adhesion strength is not easily exhibited in a
typical usable temperature area (for instance, -20 degrees C. to 40
degrees C.) of the surface protective film and thus the surface
protective film may be peeled off from the adherend. Especially, the
surface protective film is easily peeled off from an adherend having an
irregular surface such as a prism sheet.

[0041] The glass transition temperature is calculated by, for instance,
the following method.

[0042] The block copolymer composition contained in the adhesion layer of
the protective film is dissolved in a small amount of hexane and then an
excessive amount of acetone is added thereto, so that the composition is
separated into acetone-insoluble components and acetone-soluble
components. The temperature of the insoluble components is raised from a
room temperature at a ratio of 20 degrees C./min using a differential
scanning calorimeter, and a calorific value is measured to depict an
endothermic curved line. Two extended lines are added to the endothermic
curved line. Then, the glass transition temperature is obtained from the
intersection point of a half straight line between the extended lines and
the endothermic curved line.

[0043] The vinyl-polyisoprene block content is calculated by, for
instance, the following method.

[0044] A material pellet for the adhesion layer is dissolved in heavy
chloroform and, then, measurement by the NMR (nuclear magnetic resonance
spectrum method) is performed to calculate the vinyl-polyisoprene block
content from an integral intensity ratio of spectrum.

[0045] It does not matter whether or not the
vinyl-polyisoprene-block-containing styrene elastomer is hydrogenated.

[0046] Examples of the polyolefin used for the adhesion layer include
low-density polyethylene, medium-density polyethylene, high-density
polyethylene, linear low-density polyethylene, ethylene-α-olefin
copolymer, ethylene-vinyl-acetate copolymer, ethylene-ethylacrylate
copolymer, ethylene-methyl methacrylate copolymer, ethylene-n-butyl
acrylate copolymer, and polypropylene (homopolymer, random copolymer,
block copolymer). Among the above-described polyolefins, polypropylene is
preferable as a material for the adhesion layer. Polypropylene is
compatible with a styrene elastomer including a non-hydrogenated or
hydrogenated vinyl-polyisoprene block and has an excellent heat
resistance. In addition, polypropylene does not easily cause an adherend
to be contaminated by bleed.

[0047] One polyolefin or a combination of two or more polyolefins may be
used.

[0048] As a tackifier used for the adhesion layer, a resin selectively
compatible with a hydrogenated or non-hydrogenated vinyl-polyisoprene
block of a styrene elastomer can be used.

[0050] Additives such as a softener (e.g. liquid polymer and paraffin
oil), a filler, a pigment, an antioxidant, a stabilizer and an
ultraviolet absorber may be added to the adhesion layer as needed.

[0051] The adhesion layer contains the polyolefin at a mass ratio of 0.1
or more and 2 or less relative to the vinyl-polyisoprene-block-containing
styrene elastomer, and contains the tackifier at a mass ratio of 0.03 or
more and 1 or less relative to the vinyl-polyisoprene-block-containing
styrene elastomer.

[0052] When the mass ratio of the polyolefin relative to the
vinyl-polyisoprene-block-containing styrene elastomer is less than 0.1,
the adhesion strength to the adherend is too high and thus adhesive
deposits may be left after peeling. Also, since the adhesion strength to
the rear surface of the base material layer is increased, unwinding
property from the roll may be reduced. When the mass ratio of the
polyolefin relative to the vinyl-polyisoprene-block-containing styrene
elastomer exceeds 2, the adhesion strength is weak and thus the surface
protective film may be peeled off from the surface of the adherend.
Especially, the surface protective film is easily peeled off from an
adherend having an irregular surface such as a prism sheet.

[0053] The mass ratio of the polyolefin relative to the
vinyl-polyisoprene-block-containing styrene elastomer is preferably 0.15
or more and 1 or less, more preferably 0.2 or more and 0.6 or less.

[0054] When the mass ratio of the tackifier relative to the
vinyl-polyisoprene-block-containing styrene elastomer is less than 0.03,
the adhesion strength is weak and thus the surface protective film may be
peeled off from the surface of the adherend. Especially, the surface
protective film is easily peeled off from an adherend having an irregular
surface such as a prism sheet. When the mass ratio of the tackifier
relative to the vinyl-polyisoprene-block-containing styrene elastomer
exceeds 1, the adherend may be contaminated by bleed or cohesion force
may be reduced.

[0055] The tackifier content is preferably 0.05 mass % or more and 0.5
mass % or less.

[0056] The thickness of the adhesion layer is 5 μm or more and 25 μm
or less. When the thickness of the adhesion layer is less than 5 μm,
an end of an adherend having an irregular surface may be damaged. Also,
the adhesion strength may be reduced and thus the surface protective film
may be peeled off from the adherend. When the thickness of the adhesion
layer exceeds 25 μm, the adhesion strength to the adherend is too high
and thus adhesive deposits may be left after peeling. Also, since the
adhesion strength to the rear surface of the base material layer is
increased, unwinding property from the roll may be reduced.

[0057] The thickness of the adhesion layer is preferably 10 μm or more
and 20 μm or less.

[0058] The hardness (Martens hardness) of the adhesion layer is 1.0
N/mm2 or more and 2.5 N/mm2 or less. When the adhesion layer
has an extremely low hardness, i.e., a hardness less than 1.0 N/mm2,
the adhesion strength to the adherend is increased and thus unwinding
property from the roll may be reduced. When the adhesion layer has an
extremely high hardness, i.e., a hardness higher than 2.5 N/mm2, it
becomes difficult for the surface protective film to conform to the
unevenness of the adherend, so that a sufficient adhesion strength cannot
be obtained and thus the surface protective film may be peeled off from
the adherend.

[0059] The hardness of the adhesion layer is adjustable based on, for
instance, the hardnesses of the vinyl-polyisoprene-block-containing
styrene elastomer and the polyolefin, which are used for the adhesion
layer. Further, when two or more polyolefins are used, the hardness of
the adhesion layer is also adjustable based on the hardness of each
polyolefin.

[0060] In the exemplary embodiment, the base material layer and adhesion
layer are laminated by co-extrusion.

Advantages of Exemplary Embodiment

[0061] According to the above-described exemplary embodiment, the
following advantages can be attained.

[0062] The adhesion layer contains a styrene elastomer containing a
non-hydrogenated or hydrogenated vinyl-polyisoprene block according to
the exemplary embodiment. Accordingly, when a surface protective film is
rolled up as a roll, the surface protective film can be easily wound off
from the roll without applying a mold releasing treatment to the base
material layer.

[0063] Thus, efficiency of operation for attaching the surface protective
film to the surface of an adherend can be enhanced. Also, it is less
likely that the base material layer is partially stretched and deformed
or the adhesion layer is partially peeled to spoil the surface protective
film. Further, since a mold releasing treatment for obtaining unwinding
property from the roll is not required, manufacturing cost of the surface
protective film can be reduced.

[0064] The adhesion layer contains a predetermined amount of polyolefin
and tackifier while the styrene content is in the range of 5 mass % to 30
mass %, the triblock copolymer content is 80 mass % or more, and the
glass transition temperature is in the range of -40 degrees C. to 20
degrees C. in the styrene elastomer. Thus, while the surface protective
film reliably exhibits a favorable winding property from the roll, the
adhesion strength of the adhesion layer to the adherend can be adjusted
in a proper range where the surface protective film can appropriately
function.

[0065] Since problems such as peeling and adhesive deposits are not easily
occurred, the surface protective film can be favorably used for an
adherend having an irregular surface such as a prism sheet.

[0066] Since the thickness of the adhesion layer is in the range of 5
μm to 25 μm, the surface of an adherend having an irregular surface
such as a prism sheet can be favorably protected.

[0067] In other words, it is less likely that an irregular surface is
damaged because the adhesion layer is too thin, adhesive deposits are
left after peeling because the adhesion layer is too thick, or unwinding
property from the roll is reduced.

[0068] The hardness (Martens hardness) of the adhesion layer is in the
range from 1.0 N/mm2 to 2.5 N/mm2. Accordingly, when a surface
protective film is rolled up as a roll, the surface protective film can
be easily wound off from the roll without applying a mold releasing
treatment to the base material layer.

[0069] Further, it is less likely that the surface protective film has a
difficulty in extending along the unevenness of an adherend because of an
extremely high hardness of the adhesion layer, so that a sufficient
adhesion strength cannot be obtained and thus the surface protective film
may be peeled off from the adherend.

[0070] In other words, by appropriately adjusting the hardness of the
adhesion layer, the adhesion strength can be controlled. For instance,
the adhesion strength is adjustable based on, for instance, the
respective hardnesses of the styrene elastomer and the polyolefin
contained in the adhesion layer.

[0071] Since the base material layer and the adhesion layer are laminated
by co-extrusion, the base material layer and the adhesion layer can be
laminated at the same time without a solvent, thus producing a surface
protective film by a relatively simple manufacturing device. Also, due to
a simplified manufacturing process, manufacturing cost of the surface
protective film can be reduced to a moderate level. Further, in a surface
protective film formed by co-extrusion, the interlaminar strength of a
surface of the base material layer and the adhesion layer is high. In
addition, such a surface protective film hardly leaves adhesive deposits
when being peeled off from the adherend.

Modifications

[0072] The invention is not limited to the above-described embodiment but
also includes modifications and improvements as long as an object of the
invention can be achieved.

[0073] Although the surface protective film exemplarily includes the base
material layer and the adhesion layer in the exemplary embodiment, the
surface protective film may include other functional layers such as an
ultraviolet absorbing layer. In this instance, a surface of an adherend
can be protected not only from physical contact but also from other
factors such as ultraviolet rays.

[0074] The specific material, arrangement and the like described in
carrying out the invention can be altered as long as an object of the
invention can be achieved.

EXAMPLES

[0075] The invention will further be described below with reference to
Examples and Comparatives. Note that the invention is not limited to
Examples.

[0077] By co-extruding the material for the adhesion layer and BPP
(manufactured by SunAllomer Ltd., brand name: PC-684S) used as a material
for the base material layer such that the adhesion layer had an 11 μm
thickness and the base material layer had a 39 μm thickness, a surface
protective film was obtained

Example 2

[0078] A surface protective film was obtained in the same manner as in
Example 1 except for changing the respective mass ratios of RPP and LLDPE
added as polyolefins to 0.09 and 0.08.

Example 3

[0079] A surface protective film was obtained in the same manner as in
Example 1 except for changing the mass ratio of LLDPE added as a
polyolefin to 0.17 and adding no RPP.

[0081] By co-extruding the material for the adhesion layer and BPP
(manufactured by SunAllomer Ltd., brand name: PC-684S) used as a material
for the base material layer such that the adhesion layer had an 11 μm
thickness and the base material layer had a 39 μm thickness, a surface
protective film was obtained.

Comparative 1

[0082] A surface protective film was obtained in the same manner as in
Example 1 except for changing the mass ratio of RPP added as a polyolefin
to 0.17 and adding no LLDPE.

Comparative 2

[0083] A surface protective film was obtained in the same manner as in
Example 4 except for adding LLDPE as a polyolefin at a mass ratio of 0.17
and adding no RPP.

Evaluation Method

1. Adhesion Strength

[0084] Each of the surface protective films according to Examples 1 to 4
and Comparatives 1 and 2 was attached to an acrylic resin prism sheet
having a cross section shaped in a triangle having an apex angle of
approximately 90 degrees and a height of approximately 30 μm, and then
initial adhesion strength and time-dependent adhesion strength were
measured. The case where the measured resistance was in the range of
0.03N/25 mm to 0.15N/25 mm was represented by A, and the case where the
measured resistance was out of the range of 0.03N/25 mm to 0.15N/25 mm
was represented by B.

Initial Adhesion Strength

[0085] Each surface protective film was attached to an adherend (prism
sheet) under a linear pressure of 0.38 MPa at 2 m/min and stored at 23
degrees C. for 24 hours.

[0086] Then, the surface protective film was peeled off at a tensile speed
of 0.3 m/min and 180 degrees peel using a tensile testing machine, and
then a resistance was measured.

Time-dependent Adhesion Strength after Warming

[0087] The surface protective film was attached to an adherend (prism
sheet) under a linear pressure of 0.38 MPa at 2 m/min and stored at 50
degrees C. for 1 week.

[0088] Then, the surface protective film was peeled off at a tensile speed
of 0.3 m/min and 180 degrees peel using a tensile testing machine, and
then a resistance was measured.

2. Hardness (Martens Hardness) of Adhesion Layer

[0089] A Martens hardness was measured at a testing force of 1 mN and a
load speed of 0.05 mN/sec using a ultramicro hardness tester
(manufactured by Fischer Instruments K.K., HM2000S).

3. Unwinding Strength

[0090] An adhesion surface of the surface protective film was pressed to
be attached to the rear surface of the base material layer under a linear
pressure of 0.38 MPa at 2 m/min and stored at 23 degrees C. for 5
minutes.

[0091] Then, the surface protective film was peeled off at a tensile speed
of 0.3 m/min and 90 degrees peel using a tensile testing machine, and
then a resistance was measured. The case where the measured resistance
was 0.5N/25 mm or less was represented by A, and the case where the
measured resistance was more than 0.5N/25 mm was represented by B.

[0093] As obvious from the table 1, the surface protective films according
to Examples 1 to 4 using an arrangement according to an aspect of the
invention exhibited favorable adhesion strength and unwinding strength.
The adhesive force and unwinding property were stably maintained for a
long time.

[0094] Contrarily, the surface protective film of Comparative 1, in which
the Martens hardness of the adhesion layer is more than 2.5 N/mm2,
had weak initial and time-dependent adhesion strengths.

[0095] Further, the surface protective film of Comparative 2, in which the
Martens hardness of the adhesion layer is less than 1.0 N/mm2, had a
poor unwinding strength.